The present invention relates to compounds and methods for treating malignant tumors, in particular brain tumors, using such compounds.
Porphyrins in general belong to a class of colored, aromatic tetrapyrrole compounds, some of which are found naturally in plants and animals, e.g., chlorophyll and heme. Porphyrins are known to have a high affinity to neoplastic tissues of mammals, including man. Because of their affinity for neoplastic tissues, in the central nervous system (CNS), porphyrins with boron-containing substituents using boron neutron capture therapy (BNCT) can be useful in the treatment of primary and metastatic tumors. Porphyrins and other tetrapyrroles with relatively long singlet lifetimes can be used to treat malignant tumors via photodynamic therapy (PDT).
In addition, porphyrins can be used in vivo as chelating agents for (1) certain paramagnetic metal ions to achieve higher contrast in magnetic resonance imaging (MRI) or for (2) radioactive metal ions for tumor imaging in single-photon-emission tomography (SPECT) or position emission tomography (PET) and/or in radioisotope-mediated radiation therapy. Thus, appropriately radiolabeled porphyrins can be imaged noninvasively in nuclear medicine employing SPECT or PET.
Boron neutron-capture therapy BNCT! is a bimodal cancer treatment based on the selective accumulation of a .sup.10 B carrier in Lumors, and subsequent irradiation with thermalized neutrons. The production of microscopically localized high linear-energy-transfer (LET) radiation from capture of thermalized neutrons by .sup.10 B in the .sup.10 B(n,.alpha..sup.7 Li) reaction is responsible for the high efficacy and sparing of normal tissues. More specifically, the stable nuclide boron-10 (.sup.10 B) absorbs thermalized neutrons to create ionizing radiation (.sup.7 Li and .sup.4 He) with ranges of 5 and 9 .mu.m, respectively.
When BNCTI is used in patients with malignant brain tumors, the patient is injected with a boron compound highly enriched (.gtoreq.95 atom %) in boron-10. The boronated compound used is one chosen that has the property of concentrating preferentially in the brain tumor within the radiation volume. For some BNCT compounds, the action of the blood-brain barrier slows or prevents their entry into healthy, normal, surrounding central nervous system tissues. The patient's head is then irradiated in the general area of the brain tumor with an incident beam or field of epithermal (0.5 eV-10 keV) neutrons. These neutrons become progressively thermalized (average energy.apprxeq.0.04 eV) as they penetrate deeper into the head. As they become thermalized, they can more readily be captured by the boron- 10 concentrated in the tumor cells and/or tumor supporting tissues. A small proportion of the boron-10 nuclei in and around a tumor undergo a nuclear reaction immediately after capturing a neutron which produces the high LET alpha (.sup.4 He) and lithium (.sup.7 Li) particles. The tumor in which the boron-10 was concentrated is thus irradiated by these short range particles, which, on average travel a distance comparable to or slightly less than the diameter of a typical tumor cell. Therefore, a very localized, specific reaction takes place whereby the tumor receives a large radiation dose compared with that received by surrounding non-neoplastic tissues, with relatively low boron-10 concentrations.
For BNCT of malignant brain tumors, it is particularly important that there be robust uptake of boron in tumor relative to normal tissues (i.e., blood and normal brain tissues) within the neutron-irradiated target volume. BNCT has been used clinically at the Brookhaven National laboratory Medical Department using p-boronophenylalaninc BPA! as the boron carrier (Coderre, et al., 1997). BPA has the outstanding quality of not eliciting any chemical toxicity associated with its usage. However, because the brain and blood boron concentrations are approximately one-third that found in tumor, the tumor dose is restricted. In order to improve upon the currently used boron delivery agent, BPA, it has been postulated that tumor boron concentrations should be greater than 30 .mu.g B/g and tumor:blood and tumor:brain boron ratios should be greater than 5:1 (Miura, et al., 1996).
In PDT of malignant tumors using porphyrins, the patient is injected with a photosensitizing porphyrin drug. Thc drug localizes preferentially in the tumor within the irradiation volume. The patient's tissues in the zone of macroscopic tumor is then irradiated with a beam of red laser light. The vascular cells of the irradiated tumor and some of the tumor cells are rendered incapable of mitotic activity or may be rendered nonviable outright if the light penetrates the tissue sufficiently. The biochemical mechanism of cell damage in PDT is believed to be mediated largely by singlet oxygen. Singlet oxygen is produced by transfer of energy from the light-excited porphyrin molecule to an oxygen molecule. The resultant singlet oxygen is highly reactive chemically and is believed to react with and disable cell membranes. Macroscopically, there appears to be some direct damage to tumor cells, extensive damage to the endothelial cells of the vasculature, and infiltration of the tumor by macrophages. The macrophages remove detritus of dead cells from the PDT-treated zones of tissue, and in the process, are believed to damage living cells also.
In PDT, the porphyrins must be selectively retained by tumors, especially within the irradiation volume. However, the porphyrin drugs should be non-toxic or minimally toxic when administered in therapeutically useful doses. In addition, the porphyrin drugs must have absorbance peaks at long wavelengths to allow increased tissue penetration and, thereby, allow photoablation of some or all of the vasculature and/or parenchyma of deep-seated tumors.
While it is well known in medical arts that porphyrins have been used in cancer therapy, there are several criteria that must be met for a porphyrin-mediated human cancer radiation treatment to be optimized. In BNCT, the porphyrin drug should deliver a therapeutically effective concentration of boron to the tumor while being minimally toxic to normal tissues and organs at a radiotherapeutic effective pharmacological whole-body dose of porphyrin. In addition, the porphyrin should have selective affinity for the tumor with respect to its affinity to surrounding normal tissues within the irradiation volume, and should be capable of achieving tumor-to-normal-tissue boron concentration ratios greater than 5:1. We show here on the basis of in vivo studies that the latter criterion can be satisfied for brain tumors if the porphyrin, properly designed, synthesized and purified, does not penetrate the blood-brain-barrier in non-edcmatous zones of the normal CNS.
In addition, if the boron concentration and distribution in and around the tumor could be accurately and rapidly determined noninvasively, BNCT treatment planning could be more quickly, accurately, and safely accomplished. For example, this could enable neutron irradiation to be planned so that concurrent boron concentrations are maximum at the growing margin of the tumor rather than in the tumor as a whole. Thus, BNCT could be implemented by one relatively short exposure or a series of short exposures of mainly epithermal neutrons, appropriately timed to take advantage of optimal boron concentrations identified by SPECT or MRI in tumor, surrounding tissues, and blood in vivo. BNCT effectiveness in vivo is probably not diminished even when a neutron exposure is as short as 300 milliseconds. Such short irradiations have been delivered, in fact, by a TRIGA (General Atomics) reactor operating in the pulse mode. The inconvenience and discomfort to the patient of long and often awkward positioning of the head at the reactor port could be thereby ameliorated. Even this advantage alone would justify a clinical use for BNCT, if palliative results on the tumor were at least as favorable as those following the presently, available standard, 6-week, 30-fraction postoperative photon radiation therapy.
Efforts have been made to synthesize porphyrins for the diagnosis, imaging and treatment of cancer. In U.S. Pat. No. 4,959,356 issued to Miura, et al., a particular class of porphyrins was synthesized for utilization in the treatment of brain tumors using BNCT. The porphyrins described in that patent are natural porphyrin derivatives which contain two carborane cages at the 3 and 8 positions. Natural porphyrins have particular substitution patterns which are, in general, pyrrole substituted and are asymmetric. The porphyrins described in U.S. Pat. No. 4,959,356 use heme, the iron porphyrin prosthetic group in hemoglobin, as a chemical starting material; therefore, the resulting boronated porphyrins resemble heme in their basic structure. In contrast, the porphyrins of the current invention are synthetic tetraphenylporphyrin (TPP) derivatives that are symmetrically substituted at the methine positions and most are also substituted at the pyrrole positions of the macrocycle. Acyclic precursors are used as chemical starting materials so that final product yields are generally greater than those obtained from natural porphyrin derivatives.
U.S. Pat. Nos. 5,284,831 and 5,149,801 issued to Kahl, et al. describe another type of porphyrin and their uses in BNCT, PDT and other biomedical applications. Like the porphyrins described in the previous patent by Miura et al., these are also natural porphyrin derivatives but they contain four carborane cages at the 3 and 8 positions.
U.S. Pat. No. 4,500,507 issued to Wong describes a method of labeling hematoporphyrin derivatives (HPD) with .sup.99m Tc as a means of visualizing tumors using scintigraphic noninvasive imaging techniques such as SPECT. The method taught by this patent utilizes hematoporphyrin compounds that are also natural porphyrin derivatives.
U.S. Pat. Nos. 4,348,376 to Goldenberg, 4,665,897 to Lemelson, and 4,824,659 to Hawthorne teach combining labeling of an antibody with .sup.10 B and with one or more other radionuclides, including those of iodine, for purposes of imaging tumors noninvasively and thereby delineating tumor targets for exposure to thermalized neutrons. Each of these patents requires that the .sup.10 B compound be linked to a radiolabeled antibody.
It is, therefore an object of the present invention to describe a new class of non-toxic boronated porphyrin compounds used with or without ancillary substances such as antibodies. It is also an object of the present invention to provide methods for their use in treating tumors, for example, malignant brain tumors.
An additional objective of the present invention is to provide methods of utilizing any one or more of this new class of boronated porphyrins containing four carborane cages and a central metal ion to localize, analyze, and treat malignant tumors, for example, brain tumors. A further object of the present invention is to provide methods for treating malignant tumors in general using these new boronated porphyrins using BNCT and/or PDT.
Another object of the present invention is to provide a method for directly and noninvasively imaging and quantifying boron concentrations in tissues using the compounds of the present invention via SPECT, PET and/or MRI, thereby permitting rapid enhanced targeting and planning of subsequent neutron irradiation of those tissues.